1. Field of the Invention
This invention relates to welding methods for nickel or nickel alloy products having excellent corrosion resistance and high-temperature strength, clad metals covered with a layer of nickel or a nickel alloy, and double pipes with a mechanically joined inner pipe of nickel or a nickel alloy, and more particularly to gas tungsten arc welding methods therefor.
2. Description of the Prior Art
Recently the number of natural gas and oil wells exploited in highly corrosive environments and chemical plant pipelines used for the transportation of highly corrosive fluids has been increasing sharply.
Conventional carbon steel pipes, even if corrosion resistance is improved by the addition of special alloying elements or by special heat treatments, or even if corrosion inhibitors are added to the corrosive fluids they conduct, cannot prevent corrosion from the inside. Besides, they do not have high enough strength to withstand the increasing pressure involved in the transportation of fluids.
This is the reason why products of nickel or a nickel alloy (alloys containing more than approximately 15% each of chromium and nickel), clad metals overlain with a layer of nickel or a nickel alloy and double pipes with an inner pipe of nickel or a nickel alloy have found extensive use. Gas tungsten arc welding having the following advantages is commonly used with these pipes.
(1) Welding with low rates of gas dilution does not heavily impair the corrosion resistance of welded joints. PA1 (2) Few weld defects occur when welding is performed under appropriate conditions. PA1 (3) A beautiful and smooth appearance can be obtained, without requiring slag removal or bead grinding during welding.
High-nickel or high-molybdenum welding materials having a higher corrosion resistance and high-temperature strength than the base metal (the inner pipe or cladding metal in the case of a double pipe) are used. For example, Inconel 625 or Hastelloy 276 (trademarks for corrosion-resistant nickel-base alloys) are commonly chosen.
Conventional gas tungsten arc welding is so inefficient and time-consuming that construction of long pipelines has been very costly. Though recently developed high-strength clad steel pipes overlain with Incoloy 825 (a trademark for corrosion-resistant alloys) or Inconel 625 or reinforced with a mechanically joined inner pipe has realized a substantial cutback in material cost, the high construction cost mentioned above has prevented the expansion of their use.
Welding of nickel-rich alloys or high-strength clad metals overlain with such alloys with high-nickel filler metals such as Inconel 625 or Hastelloy 276 by the conventional gas tungsten arc process basically differs from that of carbon steel in the following two points.
When molten, to begin with, the alloys are more viscous and less fluid, and, therefore, does not wet the sides of the groove as much as carbon steel. Wetting of the groove sides by the molten metal is very important is welding performed in all positions, as in the on-site welding of pipelines. The interfacial tension built up by the wetting between the base metal and deposited metal supports the weight of the molten metal, thereby permitting continuous welding without causing burn-through.
While carbon steel pipes can be welded with a high deposition rate, high-nickel alloy pipes must be welded with a low deposition rate. This is the reason for the low-efficiency welding mentioned before.
The poor wetting has conventionally been improved by simultaneously causing the arc torch and filler wire to oscillate through the groove. This conventional method is designed to promote the groove sides by the deposited metal by oscillation, placing an appropriate amount of deposited metal at the most suitable point in the groove, which is, to be more specific, near the side walls thereof, and thus prevent burn-through.
Still, this oscillating method too has its limit. When the stroke of oscillation is increased, the welding speed in the middle of the groove becomes so high that an insufficient heat input and the filler wire sticking tend to occur to impair the quality of welding.
Secondly, the depth of penetration resulting from the arc welding of high-nickel alloys is smaller than that with carbon steel. Therefore, the groove in the weld zone (the height of the root face) must be reduced to between approximately 1.0 and 1.4 mm, whereas the butt weld must be made with extremely high accuracy. This is another reason for the low construction efficiency.
Filling these gaps with carbon steel is the key point in the improvement of the welding technology for products of nickel or a nickel alloy, clad metals covered with a layer of nickel or a nickel alloy, and double pipes with a mechanically joined inner pipe of nickel or a nickel-alloy. Various attempts have been made to improve the conventional gas tungsten arc welding method. For example, the welding current was increased beyond the tolerable limit to increase the wetting by the deposited metal and the oscillating speed was lowered to obtain a deeper penetration. However, these attempts have not brought about any major improvement in welding efficiency, often entailing, instead, various welding defects.